Carbon conundrum

Carbon dioxide is a crucial gas in our global ecosystem. But how we remove the excesses of carbon dioxide that human endeavours have put into the atmosphere is proving more complicated than we thought!

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Carbon dioxide is the major greenhouse gas contributing to global warming — and the major source of carbon dioxide is fossil fuels, burnt by humans to meet our energy and transport needs. Each year about 27 billion tonnes of carbon dioxide are belched out into the atmosphere.

Lucky for us, not all the carbon dioxide released into the atmosphere stays there and contributes to global warming (if it did it would be known as global sweltering). Of our annual 27 billion tonnes carbon dioxide output;

- 7 billion tonnes are absorbed by oceans;

- 7 billion tonnes are taken up by forests, and;

- 13 billion tonnes accumulate in the atmosphere each year.

However, there's a complication to this neat little equation — in the last two years, the accumulation of carbon dioxide in the atmosphere has risen much faster than expected, from an average of 13 to 18 billion tonnes. Scientists attribute this increase to humans producing more carbon dioxide and, ironically, to global warming itself — as the Earth's temperature rises it decreases the ability of oceans and forests to take up carbon dioxide.

The oceans ain't sinking like they used to!

Global warming could raise the sea levels by as much as six metres (20 feet) in the next generation and the earth could be heading for a mini ice-age. Image: Reuters

As the planet warms, the ability of our oceans to dissolve atmospheric carbon dioxide will decrease because of three key factors — the increase in water temperature, slowing of the thermohaline current and the role of phytoplankton.

The increase in water temperature will affect the solubility capacity of the oceans because colder water absorbs more carbon dioxide. This is why most carbon dioxide absorption occurs in the icy waters of the poles in the North Atlantic and Southern Oceans. As ocean temperatures rise due to global warming there will simply be less room within our oceans to absorb carbon dioxide from the atmosphere.

Global warming is also expected to apply the breaks to the thermohaline current (THC), a crucial global current that moves surface water to the ocean depths. The THC is created by cold and salty water sinking at the poles. This current drags carbon dioxide rich water deep into the ocean and brings carbon dioxide poor water to the surface where it can in turn, absorb more carbon dioxide. A slowing of this current will again result in less atmospheric carbon dioxide being removed by the oceans.

Figuring out phytoplankton

A crucial and uncertain piece in the puzzle is the role of phytoplankton. Phytoplankton are microscopic ocean plants that convert carbon dioxide to organic carbon. Fish eat the phytoplankton and the carbon is locked away from the atmosphere in the bodies of animals.

The effects of climate change on phytoplankton are not clear. On one hand more carbon dioxide may stimulate photosynthesis and encourage more phytoplankton to grow. On the other, a warmer ocean surface is thought to prevent the mixing of nutrients in the upper layer and prevent phytoplankton from flourishing.

On the whole, it's not good news. Climate models that take into account changes in water temperature, the THC and phytoplankton activity predict 14% less carbon dioxide will be absorbed by oceans by 2100.

Forest — our lungs and our sink!

Forests are often referred to as the lungs of the Earth, providing oxygen for the rest of the planet — but they are also a massive carbon sink. Plants consume carbon dioxide during photosynthesis and produce oxygen as a by-product. Plants also produce carbon dioxide during respiration, but on the whole growing plants perform more photosynthesis than respiration, leading to an overall consumption of carbon dioxide.

Carbon tied up in forests is released back into the atmosphere as carbon dioxide when soil bacteria and fungi decompose plant matter on the forest floor or when plants burn in forest fire. The highest recorded level of carbon dioxide released into the atmosphere was over 23 billion tonnes in 1998, attributed to widespread forest fires in Indonesia.

A story of many factors

Elevated carbon dioxide levels and rising temperatures have complicated effects on forests. Plants use carbon dioxide to photosynthesise and grow so it's not surprising that greater concentration of atmospheric carbon dioxide stimulates plant growth. However, not all plants respond to higher carbon dioxide in the same way, with some plants growing faster than others. Research in the Amazon rainforest has shown that the forest composition changed when trees were exposed to more carbon dioxide.

Until recently, it was assumed that forests would acts as carbon sinks; absorbing carbon dioxide pumped into the atmosphere by human activity. But a recent study showed that forests in northeast Queensland became so stressed during two dry years that they changed the way they cycled carbon. In the first dry year, they produced as much carbon dioxide as absorbed or remained carbon neutral. In the second dry year they produced a surplus of carbon dioxide. It seems that climate change may stress some forests and turn them into carbon dioxide producers.

This unexpected turnaround from carbon sink to carbon producer is because lower rainfall and increased evaporation cause photosynthesis to slow down, resulting in nett carbon dioxide production - from plant decomposition and respiration.

The effect is thought to be short-term and the forest is expected to once again become a carbon sink with a return to wetter conditions. But Australia's long-term weather forecast is for drier conditions and the role of forests as carbon absorbers may not be as reliable as the planet continues to warm.

Tangled loops

The effect of atmospheric carbon dioxide on carbon dioxide uptake systems is just one of the many feedback loops in the tangled mess of climate change. It seems the more we learn about global warming and climate change, the worse the prognosis.